Process stream sensor tubes are typified by thermowells, apparatus comprising a bimetallic temperature sensing element and an enclosing generally tubular housing, which project into piping through which process streams are being passed, for the purpose of sensing conditions, temperature in the case of thermowells, which conditions are recorded directly or converted into data reflecting temperature related conditions, e.g. flow rates between a heat source and an adjacent thermowell may be determined from temperature readings at the thermowell. Such data is used as a record or processing conditions and used to alter process kinetics according to a predetermined set of operating parameters for the process.
These sensor housings typically comprise a steel alloy tube shaped with a closed end. In use, an appropriately coupled sensing element is inserted into the tube, the tube is cantilevered into the process stream-carrying piping, and data is collected. For sensitivity of measurement, the tubes must not be too thick-walled, but many process streams contain particulates which erode the tube walls over time, a process sometimes exacerbated by the presence of aggressive chemical agents in the stream. In addition, changing reaction conditions can result in wide temperature swings, subjecting the tube walls to contraction and expansion forces generally referred to as thermal shock, which further aggravates premature wear tendencies of the housing.
Improvements in the erosion and corrosion resistance of the sensor tubes will enable longer use life, and more accurate measurements during service. It is known to improve typical sensor tube alloys, such as a 316 stainless steel by depositing an erosion resistant plasma spray coating e.g. of chromium oxide or chromium nickel boride. These coating expedients are not thermal shock resistant owing at least in part to the fact that they are merely deposited onto and not integrated with the substrate material, and thus any improvement in erosion resistance is possibly lost during use of the tube, especially where temperature swings in use alternately contract and expand the housing wall, possibly at different expansion rates than the coating.
Erosive wear as the term is used herein refers to wear caused not so much by large particle abrasion as by innumerable minute contacts by the small particles that characterize fine particulate-containing liquid and vapor streams. It has been determined that mere hardness is not indicative of successful resistance to erosive conditions, although hardness is directly related to success in abrasion resistance.
Importantly, the erosion and corrosion resistance benefits conferred by the invention can be obtained locally in areas needing them, e.g. on the leading edge of the tube, or outward from the fittings which secure the tube to the piping wall, so that the entire part need not be fabricated of costly materials, nor coated entirely with a specialized coating.